用java 编写一个程序,用于计算 x op y 这样的表达式的值，表达式从命令行输人,其中o为十，一，*，/，% 几种运算符之一。

以下是用Java编写的程序，可以计算命令行输入的表达式的值：

public class Calculator {
    public static void main(String[] args) {
        if (args.length != 3) {
            System.out.println("Usage: java Calculator x op y");
            return;
        }

        double x = Double.parseDouble(args[0]);
        String op = args[1];
        double y = Double.parseDouble(args[2]);

        double result = 0.0;
        switch (op) {
            case "+":
                result = x + y;
                break;
            case "-":
                result = x - y;
                break;
            case "*":
                result = x * y;
                break;
            case "/":
                result = x / y;
                break;
            case "%":
                result = x % y;
                break;
            default:
                System.out.println("Invalid operator: " + op);
                return;
        }

        System.out.println(x + " " + op + " " + y + " = " + result);
    }
}

运行示例：

java Calculator 2.5 + 3.7
2.5 + 3.7 = 6.2

java Calculator 4.0 - 1.5
4.0 - 1.5 = 2.5

java Calculator 5.0 \* 2.0
5.0 * 2.0 = 10.0

java Calculator 6.0 / 3.0
6.0 / 3.0 = 2.0

java Calculator 7.0 % 3.0
7.0 % 3.0 = 1.0

java Calculator 8.0 # 2.0
Invalid operator: #